1. Field of the Invention
The present invention relates to a technology for integrally building multiple electronic components in a substrate.
2. Description of the Related Art
A capacitor unit that is formed by electrically connecting outer electrodes of multiple electronic components, such as multiple ceramic capacitors, to one another such that the multiple electronic components are integrated has heretofore been known (see, for example, Japanese Unexamined Patent Application Publication No. 2009-81183, specifically paragraphs [0026] to [0029], FIGS. 3 and 5, Abstract, and other portions). If a capacitor unit is built in a built-in-component substrate, a certain number of ceramic capacitors can be built in the built-in-component substrate more easily than in the case where the same number of ceramic capacitors are individually built in the built-in-component substrate.
In addition, when a capacitor unit is formed from cheap general-purpose ceramic capacitors each having a small capacitance, the capacitor unit can function as a large-capacitance capacitor. This dispenses with the need for an expensive large-capacitance capacitor, and thus the production cost of a built-in-component substrate can be reduced. This is also advantageous in terms of efficiency because a capacitor unit having desired characteristics can be easily formed by changing the number or the type of ceramic capacitors that constitute the capacitor unit.
Referring to FIGS. 7A and 7B, an example of a method of forming such a capacitor unit 500 will be described. Firstly, a die having a cavity is prepared, and a predetermined number of ceramic capacitors 501 are contained in the cavity. Here, the ceramic capacitors 501 are contained in the cavity such that outer electrodes 502 of adjacent ceramic capacitors 501 are adjacent to or in contact with each other.
The ceramic capacitors 501 in this example are formed as so-called chip-type multilayer ceramic capacitors, and each include dielectric ceramic layers 503, inner electrodes 504 that are disposed so as to face one another via the dielectric ceramic layers 503, and outer electrodes 502 that are connected with inner electrodes 504 led out to end surfaces of the body of the ceramic capacitor 501.
Subsequently, when low-melting metal layers on the surfaces of the outer electrodes 502 are heated to a melting temperature, adjacent outer electrodes 502 are bonded to one another as illustrated in FIG. 7A, so that a capacitor unit 500 in which the multiple ceramic capacitors 501 are integrated is formed.
As illustrated in FIG. 7B, the adjacent outer electrodes 502 are integrated and electrically connected to one another, and thus the multiple ceramic capacitors 501 function as an electrically singular capacitor. In other words, the capacitor unit 500 is formed by integrating the multiple ceramic capacitors 501 into a single component and by electrically connecting the ceramic capacitors 501 to one another. FIGS. 7A and 7B illustrate an example of a conventional capacitor unit 500, in which FIG. 7A is a plan view and FIG. 7B is a cross-sectional view of the capacitor unit 500 taken along the line A-A and viewed in the direction of the arrows A.
When a component assembly, typified by the capacitor unit 500, is built in a built-in-component substrate, an electrode of the component assembly and a wiring layer of the built-in-component substrate are connected to each other using solder or a via conductor. When the component assembly is connected to the wiring layer of the built-in-component substrate by soldering, the component assembly can be built in the built-in-component substrate by being easily mounted on the wiring layer with a well-known surface-mount technology. Thus, the production cost of the built-in-component substrate can be reduced.
Generally, the same resin material is used for substrate materials (core and pre-preg) of multilayer substrates, and the same resin material is also used for multilayer substrates in which components are built. A conceivable way of reducing the production cost of a built-in-component substrate is to make the built-in-component substrate out of a general-purpose resin material, i.e., a resin material having a low glass transition temperature (Tg) or a resin material having a large coefficient of linear expansion, for reduction of the production cost of the built-in-component substrate. With this method, however, the following may occur when a built-in-component substrate on whose wiring layer a component assembly is mounted by soldering is mounted on a motherboard by soldering or when another electronic component is mounted on a surface of the built-in-component substrate by soldering. If the built-in-component substrate is heated during reflow, for example, solder with which the component assembly is mounted on the wiring layer of the built-in-component substrate re-melts and the built-in-component substrate deforms, so that the solder that has re-melted and seeped out of a gap of the built-in-component substrate, the gap having been formed due to thermal deformation, may flash or the melting solder may cause a bridge. In view of this, a built-in-component substrate that is prevented from thermally deforming has to be formed while usable resin materials are limited. This makes it difficult to reduce the production cost of the built-in-component substrate. If a built-in-component substrate is made of a resin material having a low glass transition temperature (Tg) or a resin material having a large coefficient of linear expansion, the application range of the built-in-component substrate becomes limited, for example, the produced built-in-component substrate is not capable of coping with reflow or other operations.
When a component assembly is connected to the wiring layer of the built-in-component substrate using via conductors, because of the variation in height among electronic components constituting the component assembly, via holes have to be formed in the built-in-component substrate by laser processing while an output of a laser beam used in the laser processing for forming the via holes is adjusted, and the via holes thus formed have to be filled with conductive paste or have to be subjected to via-filling plating to form via conductors. It takes time and effort to perform these operations and thus the production cost of the built-in-component substrate increases. Moreover, a difference in coefficient of linear expansion between each electronic component and the general-purpose resin material is large at the junction between each via conductor and a corresponding electronic component. Thus, a stress occurs between these members since these members have largely different coefficients of expansion at which the members expand when being heated, such as in reflow. When the stress, having thus occurred between the members attributable to the difference in coefficient of linear expansion, becomes concentrated at the junction, the reliability of connection between the electronic component and the via conductor may be compromised and poor.